1. Technical Field
This invention relates to a device for determining the position of an event inducing a signal in photodetectors, for example this position being identified with respect to the set of photodetectors. This type of position can be identified by the center of gravity of the event in a coordinate system relative to the photodetectors.
The invention is particularly applicable to determining the position of an event starting from signals output by photo-multipliers used in a gamma-camera, the position being identified with respect to the photo-multipliers themselves. A gamma-camera is a camera that is sensitive to gamma (xcex3) radiation. This type of camera is used particularly for medical imagery purposes.
2. State of Prior Art
At the present time, most gamma-cameras used in nuclear medicine operate using the principle of Anger type cameras. Document U.S. Pat. No. 3,011,057 provides further information about this subject.
Gamma-cameras have the specific feature that they display the distribution of molecules marked by a radioactive isotope previously injected into the patient, within an organ.
The structure and operation of a known gamma-camera are described and summarized below with reference to the attached FIGS. 1, 2A and 2B.
FIG. 1 shows a detection head 10 of a gamma-camera placed facing an organ 12 containing molecules marked by a radioactive isotope.
The detection head 10 comprises a collimator 20, a scintillator crystal 22, a light guide 24 and several photo-multiplier tubes 26 placed adjacent to each other so as to cover one surface of the light guide 24. For example, the scintillator may be an NaI(Tl) crystal.
The function of the collimator 20 is to select the radiation which reaches the detection head at an approximately normal incidence, among all the gamma radiation 30 emitted by organ 12. The selective nature of the collimator can increase the resolution and the sharpness of the image produced. However, the resolution is increased at the expense of sensitivity. For example, only one photon among about 10 000 xcex3 photons emitted by organ 12, is actually detected.
The xcex3 photons that passed through the collimator arrive at the scintillator crystal 22, where almost all xcex3 photons are converted into several light photons. In the rest of this text, each interaction of a gamma photon with the crystal causing a scintillation is called an event.
Photo-multipliers 26 are designed to emit an electric pulse proportional to the number of light photons received from the scintillator for each event.
In order for a scintillation event to be more precisely positioned, photo-multipliers 26 are not directly fixed to the scintillator crystal 22 but are separated from it by the light guide 24.
Photo-multipliers emit a signal, the amplitude of which is proportional to the total quantity of light produced in the scintillator by gamma radiation, in other words proportional to its energy. However, the individual signal from each photo-multiplier also depends on the distance that separates it from the point 30 at which the gamma radiation interacts with the scintillator material. Each photo-multiplier outputs a current pulse proportional to the light flux that it received. In the example in FIG. 1, small graphs A, B and C show that photo-multipliers 26a, 26b and 26c located at different distances from an interaction point 30 output signals with different amplitudes.
The position of the interaction point 30 of a gamma photon is calculated in the gamma-camera starting from signals originating from the set of photo-multipliers by taking a center of gravity weighting of the contributions of each photo-multiplier.
The principle of center of gravity weighting as used in Anger type cameras can be explained more clearly with reference to attached FIGS. 2A and 2B.
FIG. 2A shows the electric wiring of a gamma-camera detection head 10, that connects this camera to an image generation unit. The detection head comprises several photo-multipliers 26.
As shown in FIG. 2B, each photo-multiplier 26 in the detection head is associated with four resistances denoted RX31 , RX+, RYxe2x88x92 and RY+. The values of these resistances are specific to each photo-multiplier and depend on the position of the photo-multiplier in the detection head 10.
Resistances RXxe2x88x92, RX+, RYxe2x88x92 and RY+ in each photo-multiplier are connected to the output 50 of the said photo-multiplier, represented in FIG. 2B by a current generator symbol. They are also connected to common collecting rows denoted LXxe2x88x92, LX+, LYxe2x88x92 and LY+ respectively in FIG. 2A.
Rows LXxe2x88x92, LX+, LYxe2x88x92 and LY+ are in turn connected to analog integrators 52Xxe2x88x92, 52X+, 52Yxe2x88x92 and 52Y+ respectively, and through these integrators to analog/digital converters 54Xxe2x88x92, 54X+, 54Yxe2x88x92 and 54Y+ respectively. The output from converters 54Xxe2x88x92, 54X+, 54Yxe2x88x92 and 54Y+ is directed towards a digital operator 56. Rows LXxe2x88x92, LX+, LYxe2x88x92 and LY+ are also connected to a common channel, called the energy channel. This channel also comprises an integrator 57 and an analog/digital converter 58, and its output is also directed towards operator 56.
The device in FIG. 2 is used to calculate the position of the interaction according to the following equations (U.S. Pat. No. 4,672,542):   X  =                                          X            +                    -                      X            -                                                X            +                    +                      X            -                              ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢      Y        =                            Y          +                -                  Y          -                                      Y          +                +                  Y          -                    
in which X and Y are the coordinates along two orthogonal directions of the position of the interaction on the crystal, and in which X+, Xxe2x88x92, Y+, Yxe2x88x92 represent the weighted signals output by integrators 52X+, 52Xxe2x88x92, 52Y+, 52Yxe2x88x92 respectively.
The values of X and Y, and the total energy E of the gamma ray that interacted with the crystal, are established by the digital operator 56. These values are then used to generate an image, for example as described in document FR-2 669 439.
The calculation of the interaction position is affected by an uncertainty related to Poisson statistical fluctuations in the number of light photons and the number of photoelectrons produced for each event, in other words for each detected gamma photon. The standard deviation of the fluctuation reduces when the number of photons or photoelectrons increases. Due to this phenomenon, light should be collected as carefully as possible. The intrinsic spatial resolution of the camera is characterized by the width at the mid-height of the distribution of positions calculated for the same collimated point source placed on the scintillator crystal.
The resolution for gamma rays with an energy of 140 keV is usually of the order of 3 to 4 mm.
The energy of a detected gamma photon is calculated by taking the sum of the contributions of all photo-multipliers that received light. It is also affected by a statistical fluctuation. The energy resolution of the camera is characterized by the ratio of the width at the mid-height of the distribution of calculated energies, to the average value of the distribution, for the same source.
The energy resolution is usually of the order of 9 to 11% for gamma rays with an energy of 140 keV.
Finally, an Anger type gamma-camera has the advantage that it enables real time calculation of the center of gravity of photo-multiplier signals with very simple means.
The system described above has a limited number of components. Furthermore, the resistances used to inject the photo-multiplier signal in collecting rows are not very expensive.
However, this type of camera also has a major disadvantage, which is a low count rate. The count rate is the number of events, in other words the number of interactions between a xcex3 photon and the scintillator, that the camera is capable of processing per unit time.
One of the limitations in the count rate is particularly due to the fact that the camera is incapable of processing two events that take place approximately simultaneously at distinct points in the scintillator crystal.
Simultaneous but geometrically distinct events create electrical signals that are stacked in the LXxe2x88x92, LX+, LYxe2x88x92 and LY+ collecting rows and which can no longer be distinguished. These events are also xe2x80x9clostxe2x80x9d for the formation of an image.
The limitation in the count rate is not an excessive constraint in traditional medical imagery techniques. As mentioned above, the collimator stops a very large number of gamma rays and only a small number of events are actually detected.
However, gamma cameras are also used in two other medical imagery techniques in which the limitation of the count rate is an unacceptable constraint.
These techniques are called xe2x80x9ccorrection of transmission attenuationxe2x80x9d and xe2x80x9ccoincident PETxe2x80x9d (Positron Emission Tomography).
The correction of transmission attenuation technique consists of taking account of the attenuation specific to the tissue of the patient surrounding the examined organ, during the formation of a medical image. In order to determine this attenuation, the transmission of gamma radiation through the patient""s body to a gamma-camera is measured. This is done by putting the patient between a highly active external source and the gamma-camera detection head. Thus when measuring the transmitted radiation, a large number of events take place in the scintillator crystal. The large number of events per unit time also increase the probability of having several almost simultaneous events. A conventional Anger type camera is then not suitable.
The PET technique consists of injecting an element such as F18 into the patient, capable of emitting positrons. The neutralization of a positron and an electron releases two xcex3 photons emitted in opposite directions with an energy of 511 keV. The PET imagery technique makes use of this physical phenomenon, by using a gamma-camera with at least two detection heads placed on each side of the patient. The detection heads used are not equipped with a collimator. Electronic information processing, called coincidence processing, selects events that occur at the same time, and thus calculates the trajectory of gamma photons.
Therefore, detection heads are subjected to high gamma radiation fluxes. The count rate of conventional Anger type gamma-cameras is usually too limited for this type of application.
For guidance, an Anger type gamma-camera can operate normally with a detection of 1xc3x97105 events per second, although in PET imagery at least 1xc3x97106 events per second are necessary for normal operation.
Another limitation of Anger type gamma-cameras described above, is due to the fact that the calculation of the center of gravity of an event is fixed by the construction of the detection head and cannot be changed, and particularly by the choice of the resistances RXxe2x88x92, RX+, RYxe2x88x92, RY+, for each photo-multiplier. Similarly, the energy calculation is fixed by wiring photo-multipliers on a common channel (energy channel).
Therefore, devices and processes need to be developed to enable use of gamma-cameras with a high count rate.
Furthermore, it is required to develop cameras for which determination of the center of gravity or location of an event have good linearity and spatial resolution characteristics.
The purpose of the invention is a process for determination of the position P0 of an event with respect to a set of N photodetectors, this event inducing a signal in the N photodetectors, and this process comprising the following steps:
a) a step in which the signal output by each photodetector is digitized, and a value Ni,j representing the energy of the signal output by each photodetector is calculated,
b) calculate an uncorrected position P0 of the event with respect to the set of photodetectors as a function of values of Nij and the position of the photodetectors,
c) determine the distance di,j of each photodetector with respect to the position P0,
d) calculate a corrected value of Nxe2x80x2i,j=F(Nij), where F is a function that:
reduces (or does not modify) the value Nij for the photodetector corresponding to P0 and for a given number N1 of photodetectors around P0 (for example the N1 first ring photodetectors),
increases, or does not modify (or increases) the value Nij for a given number N2 of photodetectors located around the N1 previous photodetectors (for example the N2 second ring photodetectors if the N1 photodetectors are in the first ring; and first ring photodetectors, or the first and second ring photodetectors if N1=0),
tends towards 0 at higher values,
e) calculate a new position P1 of the event, as a function of the position of the photodetectors and values of Ni,j.
This type of process can be used to process digitized data, and to produce a position signal P1 of the event with respect to the set of N photodetectors.
The iterative nature of the process according to the invention gives it good spatial resolution and good linearity.
The uncorrected position P0 of the event may be calculated by calculating the coordinates (X0, Y0) of center of gravity of the said event as a function of the values of Ni,j and the position (XCi,j, YCi,j) of each photodetector.
Similarly, the position P1 of the event may be calculated by calculating the coordinates of the center of gravity (X1, Y1) of the said event as a function of the values of Ni,j and the position (XCi,j, YCi,j) of each photodetector.
Due to the choice of the function F, the process according to the invention reduces the contribution of the photodetector located facing the presumed position of the event (and possibly a number N1 of photodetectors around it); this or these photodetectors provide little information about the value of the position, or the center of gravity of the event. Furthermore, this function F assigns greater importance to photodetectors located beyond the photodetector that corresponds to the unweighted position P0 of the event and the N1 photodetectors.
For example, we could choose N1=0 or N1=8 (first ring, particularly for a square distribution of photodetectors).
One particular processing may be done for the photodetectors located at the edge of the field of N photodetectors. This processing consists of the following additional step; modify the value of the position (Xci,j, YCi,j) to a new value (XCxe2x80x2i,j, YCxe2x80x2i,j), at least one of the values |Xxe2x80x2| and |Yxe2x80x2| being greater than |X| and |Y|. Thus the weight and also the position of photodetectors close to the edge of the field will be modified; the process according to the invention simultaneously modifies their contribution and their weight in the calculation of the position or the center of gravity of the event; this enables a magnification of the field and an improvement in the linearity.
The center of gravity calculations may be done sequentially. Processing time is then longer when the number of photodetectors involved in the calculation is greater.
Alternatively, it would be possible to carry out parallel processing of the data. Thus, the coordinates (X0, Y0) of the center of gravity of the event could be determined using the following sub-steps:
b1) a sub-step in which the following are determined for each column i:
the contribution of the column to the total energy induced by the event in the set of photodetectors,
the contribution of the column to the X value of the center of gravity of the event,
the contribution of the column to the Y value of the center of gravity of the event,
b2) a sub-step in which the following are determined:
the total energy induced by the event in the set of photo-detectors,
the coordinates of the center of gravity (X0, Y0) of the event with respect to the N photo-detectors.
The process according to the invention can then be used for the operation of gamma cameras with a high count rate, which is very advantageous for the case of xe2x80x9cattenuation correction by transmissionxe2x80x9d or xe2x80x9ccoincident PETxe2x80x9d measurements. The high count rate is achieved without restricting the number of photo-detectors read. This is due to the parallelism used and the strong pipelining, in other words the sequence of simple operations. The process according to the invention can then be used to accelerate the calculation of the center of gravity of digitized contributions of photo-detectors by having this calculation carried out in parallel.
The same parallel processing may be applied to determine the coordinates of the center of gravity (X1, Y1) of the event, with the same advantages.
Obviously, columns could be replaced by rows within this invention, the calculation principles being unchanged.
It would be possible to carry out a preliminary step to detect the presumed position of an event. In this case, a subset of Nxe2x80x2 photo-detectors among the N photo-detectors could be delimited around this presumed position, only the signals from these Nxe2x80x2 photo-detectors being used to carry out steps b, c, d and e in the process according to the invention.
The invention is applicable particularly advantageously when the photo-detectors are photo-multipliers in a gamma-camera. In particular, imagery techniques in correction of a transmission attenuation, and coincident PET imagery techniques, could be used.
Another purpose of the invention is a device for embodiment of the process described above.